This invention relates to a binary diffractive optical element, and, more particularly, to a method for correcting non-uniform diffraction efficiency in a binary diffractive optical element.
The propagation of a light beam can be changed by three basic means: reflection by a mirror, refraction by a lens and diffraction by a grating. Optical systems traditionally rely on reflection and refraction to achieve desired optical transformations. Optical design, based on mirror and lens elements, is a well-established and refined process. Until recently, the problems with diffraction and fabricating high efficiency diffractive elements have made diffractive elements unfeasible components of optical systems.
The diffractive process does not simply redirect a light beam. Diffraction, unlike refraction and reflection, splits a light beam into many beams--each of which is redirected at a different angle or order. The percentage of the incident light redirected by the desired angle into some given diffraction order is referred to as the diffraction efficiency for that order. The diffraction efficiency of a diffractive element is determined by the element's surface profile. If the light that is not redirected by the desired angle is substantial, the result will be an intolerable amount of scatter in the image or output plane of the optical system.
Theoretically, on-axis diffractive phase elements consisting of a grating having a given period can achieve 100 percent diffraction efficiency. To achieve this efficiency, however, a continuous phase profile within any given period is necessary. The theoretical diffraction efficiency of this surface profile is also relatively sensitive to a change in wavelength. By contrast, refractive and reflective elements are relatively wavelength insensitive. The technology for producing high quality, high efficiency, continuous phase profiles of the diffraction does not presently exist.
A compromise that results in a relatively high diffraction efficiency and ease of fabrication is a multi-level phase grating. The larger the number of discrete phase levels, the better the approximation of the continuous phase function. The multi-level phase surface profiles of the grating can be fabricated using standard semiconductor integrated circuit fabrication techniques.
As disclosed in Binary Optics Technology: The Theory and Design of Multi-level Diffractive Optical Elements by G.J. Swanson of the Lincoln Laboratory at the Massachusetts Institute of Technology, (Technical Report 854, 14 Aug. 1989), herewithin incorporated by reference, and the resulting U.S. Pat. No. 4,895,790, a fabrication process starts with a mathematical phase description of a diffractive phase profile and results in a fabricated multi-level diffractive surface. The first step is to take the mathematical phase expression and generate from it a set of masks that contain the phase profile information. The second step is to transfer the phase profile information from the masks into the surface of the element specified by the lens design.
The first step involved in fabricating the multi-level element is to mathematically describe the ideal diffractive phase profile that is to be approximated in a multi-level fashion. The next step in the fabrication process is to create a set of lithographic masks which are produced by standard pattern generators used in the integrated circuit industry.
A substrate of the desired material, such as Ge, ZnSe, Si, GaAs, and SiO.sub.2, is coated with a thin layer of photoresist. A first lithographic mask is then placed in intimate contact with the substrate and illuminated from above with an ultraviolet exposure lamp. Alternately, pattern generators, either optical or electron beam, can expose the thin layer of photoresist. The photoresist is developed, washing away the exposed resist and leaving the binary grating pattern in the remaining photoresist. This photoresist will act as an etch stop.
The most reliable and accurate way to etch many optical materials is to use reactive ion etching. The process of reactive ion etching anisotropically etches material at very repeatable rates. The desired etch depth can be obtained very accurately. The anisotropic nature of the process assures a vertical etch, resulting in a true binary surface relief profile. Once the substrate has been reactively ion etched to the desired depth, the remaining photoresist is stripped away, leaving a binary surface relief phase grating.
The process may be repeated using a lithographic mask having half the period of the first mask. The binary phase element is recoated with photoresist and exposed using the second lithographic mask which has half the period of the first mask. After developing and washing away the exposed photoresist, the substrate is reactively ion etched to a depth half that of the first etch. Removal of the remaining photoresist results in a 4 level approximation to the desired profile. The process may be repeated a third and fourth time with lithographic masks having periods of one-quarter and one-eighth that of the first mask, and etching the substrates to depths of one-quarter and one-eighth that of the first etch. The successive etches result in elements having 8 and 16 phase levels. More masks than four might be used, however, fabrication errors tend to predominate as more masks are used.
This process produces a multilevel surface relief grating structure in the substrate. The result is a discrete, computer-generated structure approximating the original idealized diffractive surface. For each additional mask used in the fabrication process, the number of discrete phase levels is doubled, hence the name "binary" optical element or, more precisely, a binary diffractive optical element.
After only four processing iterations, a 16 phase level approximation to the continuous case can be obtained. The process can be carried out in parallel, producing many elements simultaneously, in a cost-effective manner.
An ideal sixteen phase level structure achieves 99 percent diffraction efficiency. The residual 1 percent of the light is diffracted into higher orders and manifests itself as scatter.
In practice, however, the actual efficiencies of actual binary diffractive optical elements is less than ideal due to fabrication errors, particularly in mask-to-mask registration and alignment errors. In many optical systems, these effects result in an untolerable amount of scatter.
After the first etching step, the second and subsequent lithographic masks have to be accurately aligned to the existing pattern on the substrate. Alignment is accomplished using another tool standard to the integrated circuit industry, a mask aligner.
As noted, the photoresist on the substrate can be exposed with an electron-beam pattern generator. The e-beam direct-write process eliminates masks and their corresponding alignment and exposure problems. Binary optics have also been reproduced using epoxy casting, solgel casting, embossing, injection molding and holographic reproduction.
Binary optical elements have a number of advantages over conventional optics. Because they are computer-generated, these elements can perform more generalized wavefront shaping than conventional lenses or mirrors. Elements need only be mathematically defined: no reference surface is necessary. Therefore, wildly asymmetric binary optics are able to correct aberrations in complex optical systems, and elements can be made wavelength-sensitive for special laser systems.
The diffractive optical elements are generally thinner, lighter and can correct for many types of aberrations and distortions. It is possible to approximate a continuous phase profile with a stepwise profile of discrete phase levels.
Diffractive optical elements approximated by etching binary levels in substrates using several masks commonly suffer an efficiency non-uniformity induced by fabrication errors.
In particular, when mask alignment errors are present, the approximation desired surface relief phase grating structure is degraded. Any lack of uniformity to an ideal multiple level surface relief phase grating structure will reduce the efficiency of the desired diffraction order and will scatter light energy into undesireable angles. For any given absolute alignment error of one of the masks, the relative alignment error relative to grating pitch or period is greatest at the extremes of a lens element where the grating period is smallest. For a diffractive optical element, a given misalignment of one of the masks will result in a loss of diffraction efficiency which may be small at the optical element center but will increase in proportion to distance from the center along some radius. Additional mask alignment errors will generate further inefficiencies but along other radii. Taken together, mask alignment errors will result in a random pattern of diffraction inefficiencies. Energy loss will, in general, be greatest near the optical element periphery.
Often a master binary diffractive optical element is fabricated on an IC fabrication line. This master will then be replicated for volume production. The diffraction inefficiency pattern will be fixed. In these cases, the pattern of inefficiency can be measured and a single compensation designed and utilized.
Even in perfectly aligned masks, the diffraction inefficiency from not achieving an actual continuous phase profile will vary with the radius of the optical element. In this case, the difference between the binary approximation of the continuous function would be a sawtooth pattern having a large pitch at the optical element center and decreasing pitch away from center. This sawtooth pattern difference acts as a variable grating which scatters light into many diffraction orders and away from the desired angle, scattering angles being greatest near the optical element periphery.
In the case of spherical diffractive optical elements, each grating is a circle about the optical element center and constitutes a Fresnel zone. In the binary approximation, the Fresnel zone becomes a series of circular steps.
The diffraction efficiency for a specific binary diffractive optical element will be dependent upon specific mask alignment errors during fabrication and cannot be determined prior to fabrication but can be measured following fabrication. In typical high volume replication, all diffraction inefficiency patterns will be identical in each copy and a common compensating corrector can be fabricated.
It is an object of this invention to provide a means to correct the non-uniform diffraction efficiencies of a binary diffractive optical element.
It is another object of this invention to provide a second two-level binary diffractive optical element which when in the optical path with a first binary diffractive optical element provides a uniform diffraction efficiency to a beam diffracted and attenuated by the two binary diffractive optical elements.